A typical disc drive includes a housing that encloses a variety of disc drive components. The components include one or more rotating discs having data surfaces that are coated with a medium for storage of digital information in a plurality of circular, concentric data tracks. The discs are mounted on a spindle motor that causes the discs to spin. Each rotating disc has a corresponding head gimbal assembly (HGA). The HGA includes a slider, which carries a transducer that writes information to and reads information from the data surfaces of the discs. The slider and transducer are often together referred to as the “head.” The HGA also includes a gimbal that allows the slider to pitch and roll while following the topography of the disc. An actuator mechanism moves the HGAs from track to track across the surfaces of the discs under control of electronic circuitry. The actuator mechanism includes a track accessing arm and a suspension for each HGA. The suspension includes a load beam. The load beam provides a preload force, which forces the slider toward the disc surface.
During operation, as the discs rotate, the discs drag air under the respective sliders and along their bearing surfaces in a direction approximately parallel to the tangential velocity of the discs. As the air passes beneath the bearing surfaces, air compression along the air flow path causes the air pressure between the discs and the bearing surfaces to increase, creating a hydrodynamic lifting force that counteracts the load force provided by suspensions. The hydrodynamic lift force causes the sliders to lift and fly above or in close proximity to the disc surfaces.
In a magnetic recording system it is desired to keep the magnetic head at a known constant distance from the magnetic medium surface in order to meet overall system performance and reliability measures. For this purpose, air bearing designs should take into account the given head and media specifications to compensate any deviations from the desired height. However, the magnetic head does not always fly over the medium of interest with a desired Head-Media Spacing (HMS), but rather deviates from this desired value. There are two main components of HMS deviation from the desired value.
Static HMS deviations result from manufacturing variations in head and media combinations. In general, each head will fly at a different average height over the medium. The average fly height is also a function of the radius at which the head is flying. For a given radius, the difference between the mean fly height of any head/media pair and the desired HMS is defined as static HMS variation.
Dynamic HMS deviations cause the HMS to vary about the mean fly height due to factors such as compressibility of the air bearing, asperities on the medium, excitation of the suspension, and gimbal modes on which the head is mounted, etc. Dynamic HMS variation is defined as the variation in fly height about the mean fly height for a given head and medium combination at a given radius.
In one conventional recording system, the mean static HMS was measured at 17.05 nm, with a standard deviation of 0.34 nm. This variation is large enough to cause poor system performance and reliability in some production line samples. The HMS values for those samples can be detected, and a compensation mechanism can be applied to those samples to correct for deviations from the desired HMS. A known compensation mechanism is based on applying heat to the write head prior to writing in order to cause the pole tip of the writer to protrude from the head to achieve the desired static HMS. That technique requires heat that is produced using the preamp in the data storage system to power a heater on the head. The amount of heat to be applied as a function of the disc radius is determined during a factory calibration routine.
In conventional (longitudinal and perpendicular) magnetic recording, whenever the applied field is larger than the coercivity (Hc) of the medium, the medium will be magnetized towards a +Mr (positive remanent magnetization) direction (i.e., magnetized left or up), and similarly if the applied field is smaller than −Hc the magnetization will be towards a −Mr direction (i.e., magnetized right or down).
However, the conventional magnetic recording architectures are limited by well-known super paramagnetic limits. Heat Assisted Magnetic Recording (HAMR) uses a medium with very high coercivity Hc to make sure that the medium is thermally stable with very small grain volumes V. The coercivity is reduced during the write process by heating the medium, for example with a focused laser beam. Once the medium is heated, the reduced coercivity makes writing possible. Then, after writing the bit, the medium cools back to its original temperature with high coercivity H, allowing the medium to be thermally stable.
There is a need for a HMS compensation method that can be applied to heat assisted magnetic recording.